Optical sheets used for display device surfaces have layers with functions such as an anti-glare property, antistatic property and antifouling property, laminated as functional layers on the observer side of a transparent base material. To exhibit these functions, in order to impart an anti-glare property for example, methods of forming irregularities on the surface layer or adding diffusion particles to the resin forming the surface layer are employed. Conductive fine particles or a conductive resin may be added to impart an antistatic property, or a fluorine-containing polymer or stain-proofing agent may be added in order to impart an antifouling property.
Since such diffusion particles, conductive fine particles and additives are not completely phase-soluble with surface layer-forming resins, an optical sheet that employs them has a function of diffusing visible light. The irregular sections of the surface layer also have the function of diffusing visible light.
In addition, irregularities larger than the visible light wavelength range are formed in the surface layer, the transparent base material back side and between each layer in order to prevent interference patterns between optical sheets and interference patterns between optical sheets and display devices, and such irregularities also have the function of diffusing visible light.
According to the invention, such causes of visible light scattering are defined as “diffusion factors”, and the presence of such diffusion factors causes the anti-glare sheet for a liquid crystal display device to have reduced contrast due to reflection of external light. In other words, an anti-glare sheet for a liquid crystal display device should maintain the function of the optical sheet while preventing loss of contrast.
Among the aforementioned diffusion factors, the haze exhibited by surface irregularities is defined as the “surface haze” while the haze exhibited when smoothing has been performed using the resin that forms such surface irregularities, or a resin with a difference in refractive index of at least 0.02 from such a resin, is defined as the “internal haze”, and these are measured according to JIS K 7136.
The haze value, or the ratio of the internal haze and total haze, is commonly used as a simple method for evaluating contrast. Specifically, it has been considered that an optical sheet with low contrast reduction can be produced by specifying the materials and controlling the production conditions in the optical sheet production process for a lower haze value (see Patent documents 1-3).
However, contrast often differs even with the same haze value, and it has been found that, even with production using the haze value and the ratio of the internal haze and total haze as indexes, for example, it is not always possible to stably produce a satisfactory anti-glare sheet for a liquid crystal display device.
In addition, it has been attempted to lower the reflectance by additionally providing a low-refraction interference layer on the anti-glare layer, but this requires precise formation of a film of about 100 nm, and is extremely expensive.
In recent years, moreover, with the widening use of delivery systems including “one seg”, it has become increasingly possible to view both still images and dynamic images on the same display. Opportunities are also increasing for film appreciation, whereby images are enjoyed in prepared monitoring environments. The image quality demanded for display terminals has therefore also undergone a change, leading to demand for development of an anti-glare sheet for a liquid crystal display device with excellent suitability for combinations of still images and dynamic images, and notably excellent high levels of blackness in dark surroundings.
As illustrated by Patent documents 4 and 5, for example, different performance is required for still images and dynamic images, while visual observation environments of observers also differ.
As a result of diligent research on such conflicting problems, the present inventors have found that the balance between internal diffusion and surface diffusion that has been considered in the prior art does not account for the total haze, but that in addition to the internal diffusion and surface diffusion, the total haze is affected by how these two diffusion factors are related to each other in the anti-glare sheet.
The present inventors have also found, as a result of conducting diligent research on the performance required by anti-glare sheets for liquid crystal display devices for high blackness in dark surroundings and for dynamic images and still images, that in order to obtain a high level of blackness in dark surroundings it is necessary to exhibit diffusion properties of a nature such that virtually no “stray light component” is produced, which has not been considered in the past, and the “stray light component” must be considered for projected light of dynamic images and still images, and that in order to obtain image quality worthy of appreciation it is important to consider the stray light component while providing a suitable regular reflection component, for which only prevention has been considered in the past.
In other words, as regards stray light due to internal diffusion, when dark sections (for example, black) and light sections (for example, white) are present in the same screen, projected light in the light sections is partially presented as stray light due to diffusion factors inside the optical film and surface irregularities (surface diffusion factors), while “flares”, or light emitted from dark sections (light of the image that is reflected or diffused in a complex manner, due to various factors inside the image display device, before exiting the surface of the image device, thereby failing to be directed to the ideal location for the image output such that the image on the surface of the image device becomes partially whitened, appearing as blurred light and reducing the image quality) not only cause reduced contrast, and especially reduced contrast in dark surroundings, but also eliminate the stereoscopic visual effect and result in a planar, featureless image. The stray light is minimally affected on the front face, but is more powerfully affected in oblique directions.
In regard to the regular reflection component, it was found that an optical film with extremely low regular reflection prevents clear presentation of images and results in their perception as simulations, whereas an optical film with an appropriate regular reflection component presents clear images and tends to result in their perception as actual objects, increasing the unique gloss and brightness of images on a dynamic image screen, to produce images with a sense of motion.
The performance that includes contrast, a stereoscopic visual effect and motion, that are required for such dynamic images (for example, for a scene with a youth under a blue sky, the black hair displayed on the screen is smooth black allowing each individual hair to be visible, while the black pupils are moist black, and the skin is visible in the vivid brilliance characteristic of youth) will be referred to as “vivid complexion and blackness”.
Still images must have excellent contrast and prevention of unwanted reflection, and such performance of contrast and prevention of unwanted reflection required for still images will be referred to as “image crispness”. In other words, anti-glare sheets for liquid crystal display devices with excellent vivid complexion and blackness and image crispness are desired.
Moreover, in recent years, demand has been increasing for anti-glare sheets with excellent “blackness in dark surroundings”, which is a degree of notable, high-level blackness under modern conditions for appreciation, such as for film appreciation, or in other words, appreciation under dark surrounding conditions without external light.
Conventional evaluation of image quality has included the “black tightness” mentioned in Patent document 6 and “glazed black feel”, mentioned in Patent document 7.
In order to improve narrowness of angle, which is a fundamental defect in liquid crystal displays, anti-glare sheets are often provided with diffusibility. However, providing diffusibility can lower contrast, especially for frontal viewing.
Black tightness is evaluated as a compromise between viewing angle enlargement and contrast, and comparing blackness during power-off and blackness during power-on (black images) directly from the front of the display, with a more intense blackness being evaluated as a more powerful tight feel for the screen.
Due to the system structure of a liquid crystal display, light leaking from the liquid crystal display unit itself (leaked light) is present even during black display, and therefore the blackness during power-on, as seen directly from the front, is the level of blackness resulting from a combination of this leaked light and external light reflection, while blackness during power-off is the blackness with only from external light reflection, since no projected light is present.
Stated differently, “black tightness” means an intense level of blackness against both external light and leaked light, without consideration of stray light, unlike the aforementioned vivid complexion and blackness, and also without consideration of an appropriate necessary level for the regular reflection component, and therefore even if the contrast is high, the gloss and brightness of the image is inferior, no motion is produced, and the vivid complexion and blackness is not increased.
Also, “glazed black feel” is black reproducibility when an image display device displays black in a light environment, i.e., abundant expression of graded black. The measurement is performed after attachment to a cross nicol polarizing plate or a black acrylic board via an acrylic pressure-sensitive adhesive for optical films (product with total light transmittance: ≧90%, haze: ≦0.5%, film thickness: 10-55 μm, such as the MHM Series by Nichiei Kakoh Co., Ltd., or “L8010” by Hitachi Chemical Co., Ltd.) on the side opposite the film side of an optical laminate, by visual evaluation under three band fluorescence. That is, with this measurement method, evaluation of moving images is not performed and the effect of stray projected light is completely ignored. Therefore, even with high gloss and brightness, no dark surrounding contrast or stereoscopic visual effect is produced, and the vivid complexion and blackness is not increased.
“Contrast” is the ratio of white luminance to black luminance, and since the absolute value of black luminance is small, the effect on contrast is greater. In order to obtain images with excellent contrast, it is necessary to have excellent “black tightness”, as the improved viewing angle blackness, “blackness in dark surroundings” as the absolute blackness, and “glazed black feel” as abundant graded expression in the black region (hereinafter referred to as “excellent black reproducibility”).
Also, in order to present both still images and moving images, it is necessary to exhibit excellence in terms of both vivid complexion and blackness with a stereoscopic visual effect and sense of motion, and image crispness.
With the inventions described in Patent documents 8 and 9, which limit the diffusion property of the anti-glare sheet, the contrast is satisfactory, but no consideration is given to the issues of physical performance including adhesiveness and hard coat properties, which are indispensable for practical use, or glare and presentation of both moving images and still images, and therefore sufficient performance has not been exhibited.